CASTLE ROCK, Colo. (Reuters) - Two teenagers accused of fatally shooting a classmate and wounding eight others at a Denver-area high school last week were formally charged on Wednesday in a case that the presiding judge has put under seal.

Devon Erickson, 18, and Alec McKinney, 16, are accused of opening fire on fellow students in two classrooms at the Science, Technology, Engineering and Math (STEM) charter school in Highlands Ranch, Colorado, on May 7. McKinney, who identifies as male, was listed on the court docket as Maya Elizabeth McKinney.

Douglas County District Judge Theresa Slade has put the charges along with the entire case file under seal, banning the public from seeing it. But during Wednesday's hearing in open court, it was disclosed that McKinney has been charged with first-degree murder and is being tried as an adult.

Both Erickson and McKinney, who are being held without bond, were arrested on suspicion of a single count of first-degree murder and 29 counts of attempted murder immediately after the attack.

Denver’s ABC affiliate television station has reported that the two pistols used in the attack were stolen from the home of Erickson, whose parents had purchased the guns legally. The Douglas County Sheriff's Office has declined to comment.

MEMORIAL SERVICE

The student killed in the attack, Kendrick Castillo, is to be honored a memorial service on Wednesday at a Highlands Ranch church. The 18-year-old robotics enthusiast and aspiring engineer was shot dead three days before graduation when he and two other students charged the shooters in an effort to disarm them, authorities said.

The attack occurred less than a month after the 20th anniversary of the Columbine High School massacre in nearby Littleton, in which two students shot 13 people to death before committing suicide.

Five months ago, a school district official urged the STEM school’s director to investigate allegations of student bullying and violence by a parent who feared they could lead to the next “Columbine.” The director said an investigation found no evidence to support the allegations.

The STEM school had no sworn police officer at the 1,850-student campus, after a dispute with the sheriff’s office over the previous school resource officer’s role ended that relationship last year, the school said last week.

Instead a private security company was hired to patrol the kindergarten-through-high-school campus. An armed security guard responded to last week’s shooting.

ABC News, citing an unnamed law enforcement official, reported last week that the security guard may have mistakenly fired on sheriff’s deputies called to the scene and wounded a student in the chaos.

The Douglas County District Attorney office said on Wednesday it had turned over the investigation into the security guard's actions to a special prosecutor from neighboring El Paso County. A spokeswoman for El Paso County District Attorney Dan May declined to comment.